Optical fibers or fiber bundles have been terminated in ferrules to facilitate handling of the relatively delicate fibers so that they may be readily connected either to other fibers or to active devices such as light emitters or light detectors. In general, because the fibers or bundles vary in diameter, providing an adequate ferrule is difficult. The problem of the non-uniformity of the fiber diameter is accommodated in "rigid" ferrules by either tailoring the size of the aperture in the end of the ferrule to the fiber, or by adhesively attaching the end of the fiber to a slightly oversized aperture in the end of the ferrule. The adhesive attachment involves potting of the fiber and polishing its end which is time consuming and inconvenient. Providing an aperture tailored to a given fiber is impractical, and providing an oversized ferrule aperture can result in intolerable eccentricities of the fiber in the finished ferrule.
In order to accommodate fibers of varying diameter, "resilient" ferrule connectors have been utilized in which oversized apertures are used. Either the tip of the ferrule is made of resilient material with an oversized aperture through the tip, or a resilient conical insert having an oversized aperture is inserted in a solid outer ferrule. In the first case, the ferrule tip is compressed about a fiber passing through the tip by inserting the tip into an alignment sleeve or by providing a crimp shell around the tip. In the second case, the insert is pressed into the solid outer ferrule after fiber insertion, whereby the insert is compressed around the fiber.
More specifically, resilient ferrule connectors of the aforementioned insert type have been utilized in the past for loosely buffered cables, as exemplified by U.S. Pat. No. 4,190,317 in which a resilient insert is positioned within the nose of a ferrule and is forced rearwardly so as to compress about the fiber optic cable and retain it concentrically in the nose. It will be appreciated that in so doing, the optical fiber is pushed rearwardly with the insertion of the terminating insert. As a result this type of connector is not suitable for tightly buffered fiber optic cables since the fiber in this type of cable is not free to move rearward, and the fiber may be fractured when the insert is forced into the nose of the ferrule.
It should be noted that optical fibers are generally carried in cables, either in a loosely buffered or tightly buffered state. Tightly buffered cables are more difficult to terminate since they may break during the terminating process. For tightly buffered optical fibers the clad fiber is surrounded by a relatively thick coating which is in turn surrounded by a braided shield, usually of Kevlar, for tensile strength. The Kevlar shield is in turn surrounded by a cable jacket. In this manner a relatively rigid optical fiber is secured to the cable jacket and is tightly held thereby. On the other hand, loosely buffered cables include a loosely-held clad fiber which has a very thin coating, mainly utilized to prevent abrasion of the outer surface of the clad fiber. This fiber is carried loosely in a jacket with space between the fiber and the jacket. In general, the fiber is longer than the jacket and is coiled within the jacket to permit axial movement of the fiber relative to the jacket. It should be noted that while loosely buffered fiber optic cables are used generally in the communications field, tightly buffered cables are usually used for information transfer within computer networks.
There does exist a class of resilient ferrule connectors suitable for use with both loosely buffered and tightly buffered cables. As exemplified by U.S. Pat. No. 3,999,837, the connector utilizes a simple hollow resilient ferrule through which an optical fiber is passed. This resilient ferrule is merely inserted into an alignment sleeve which both compresses the ferrule around the optical fiber and aligns it to an opposing ferrule. However, with connectors of this type, the ferrule is not compressed onto the fiber unless mated in an alignment sleeve. Thus, in an unmated condition the ferrule does not securely grip the fiber. In general, this connector requires epoxy and a pot and polish operation in which a fiber with an epoxy coating is inserted into the ferrule and the ferrule is inserted into a sleeve until the epoxy hardens. After hardening, the fiber is cut off at the tip. The tip is then polished smooth. Not only are pot and polish operations inconvenient, failure of the epoxy can occur if the ferrule is unmated for any length of time.
As illustrated in U.S. Pat. No. 4,127,319 a resilient ferrule is terminated with a sleeve or shell which is crimped onto the nose portion of the ferrule. With the crimping on of the sleeve, the optical fiber is secured to the nose of the ferrule and is maintained loosely concentric to the finally crimped outer dimensions of the crimped shell or sleeve. It will however be appreciated that the utilization of a crimp sleeve can result in misalignments due to the variability in the outer dimensions of the crimp sleeve after having been crimped. While the crimp sleeve generally takes on the configuration of the crimping device, due to the deformable nature of the crimp shell itself the outer dimensions of the crimp shell can vary greatly. When a ferrule terminated in this manner is inserted into an alignment sleeve the variability in the outer dimension of the crimp shell affects the coaxiality or concentricity of the fiber vis a vis the alignment sleeve.
By way of further background, U.S. Pat. No. 4,090,778 describes the use of an oversized watch jewel at the tip of a rigid ferrule to facilitate accurate ferrule alignment in a sleeve, with the outer dimension of the watch jewel providing an accurate surface for the alignment of the ferrule within the alignment sleeve.